Populations and Succession

Exponential (Geometric) Growth

• Occurs when resources are unlimited
• Population doubles at regular intervals
• J-shaped curve on a graph
• Occurs in laboratory conditions or when a species colonises a new habitat

Logistic Growth (Sigmoid/S-curve)

In reality, growth follows an S-shaped curve with phases:

1. Lag phase: Small population; slow growth as organisms adapt or reproduce initially
2. Log (exponential) phase: Rapid growth; abundant resources, few limiting factors
3. Stationary phase: Growth slows and levels off as the population reaches the carrying capacity ($K$)
4. The population fluctuates around $K$

Carrying Capacity ($K$)

• The maximum population size that an environment can sustainably support
• Determined by the availability of resources (food, water, space, light) and the impact of limiting factors
• When population exceeds $K$: resources become scarce → death rate increases, birth rate decreases → population falls back to $K$

Abiotic Factors

• Temperature, light, water availability, oxygen levels, pH, soil nutrients
• These are density-independent — affect the population regardless of its size

Biotic Factors

• Intraspecific competition: Competition within the same species (density-dependent — intensifies as population grows)
• Interspecific competition: Competition between different species for the same resources
• Predation: Predator-prey relationships produce cyclical population fluctuations
• Disease: Spreads more easily in dense populations (density-dependent)
• Parasitism: Can weaken and reduce populations

Predator-Prey Cycles

• Prey increases → predator food supply increases → predator numbers increase
• More predators → prey decreases → predator food decreases → predator numbers decrease
• Fewer predators → prey recovers → cycle repeats
• Predator curve lags behind the prey curve

Succession is the process by which communities of organisms colonise an area and change over time, ultimately reaching a stable climax community.

Primary Succession

Occurs on bare, lifeless surfaces (no soil initially):

• Examples: Bare rock, volcanic lava, sand dunes, new islands

Stages:

1. Pioneer species colonise the bare surface (e.g., lichens, mosses, algae)
   • These are hardy species that can survive extreme conditions
   • They begin to break down rock and, when they die, their remains add organic matter
2. Soil begins to form — weathered rock + dead organic matter (humus)
3. Small plants (grasses, ferns) can now grow in the thin soil
4. As soil deepens, larger plants (shrubs, small trees) establish
5. Eventually, large trees and complex communities develop
6. The final stable community is the climax community — it remains relatively unchanged unless disturbed

Secondary Succession

• Occurs on land where a community has been destroyed or disturbed but soil remains (e.g., after fire, farming, deforestation)
• Progresses faster than primary succession because soil and seed banks already exist
• Follows the same pattern: pioneer species → intermediate communities → climax community

Key Features of Succession

Deflected Succession (Plagioclimax)

• Human activity (mowing, grazing, burning) can prevent succession from reaching its natural climax
• The community that develops is a plagioclimax — maintained by continued human intervention
• Example: Grasslands in the UK are maintained by sheep grazing; without grazing, they would succeed to woodland

Quadrats

• Used for sessile (non-moving) organisms — plants, slow-moving invertebrates
• Random sampling: use random coordinates to place quadrats
• Population estimate: $N = \text{Mean per quadrat} \times \frac{\text{Total area}}{\text{Quadrat area}}$

Transects

• Used to study distribution along an environmental gradient
• Line transect: Record species touching the line at intervals
• Belt transect: Place quadrats at intervals along the line

Mark-Release-Recapture (Lincoln Index)

• For mobile animals $N = \frac{M \times S}{R}$ Where: $M$ = marked and released, $S$ = second sample total, $R$ = recaptured marked individuals

Assumptions:

• No births, deaths, immigration, or emigration between samples
• Marking doesn't affect survival or behaviour
• Marked individuals mix fully with the population
• Marks don't wear off

Biodiversity can be quantified using Simpson's Index of Diversity:

$D = 1 - \sum \left(\frac{n}{N}\right)^2$

Where:

• $n$ = number of individuals of each species
• $N$ = total number of all individuals
• $D$ ranges from 0 (no diversity) to 1 (infinite diversity)
• Higher $D$ = greater diversity

Example Calculation

A habitat has: Species A = 20, Species B = 15, Species C = 5. Total $N = 40$.

$D = 1 - \left[\left(\frac{20}{40}\right)^2 + \left(\frac{15}{40}\right)^2 + \left(\frac{5}{40}\right)^2\right]$ $= 1 - [0.25 + 0.140625 + 0.015625] = 1 - 0.40625 = 0.594$

Question: Describe the process of primary succession on bare rock and explain how it leads to a climax community. (6 marks)

Solution:

Primary succession begins when pioneer species such as lichens colonise bare rock. Lichens can survive with minimal nutrients and water. They produce acids that slowly weather the rock, breaking it into small particles. When pioneer species die, their remains are decomposed, adding organic matter (humus) to the rock particles, forming a thin layer of soil. This soil allows mosses and then small plants like grasses to grow. Their roots further break up the rock and their dead material adds more humus, deepening the soil. As soil depth and quality increase, larger plants (shrubs and eventually trees) can establish. Each stage (seral stage) modifies the environment, making it more suitable for the next set of species and less suitable for the current ones. Eventually, a stable climax community develops (e.g., deciduous woodland in temperate UK). This community is in equilibrium with the environment and remains relatively stable unless disturbed.

• Populations grow logistically (S-curve) in limited environments, stabilising around the carrying capacity ($K$).
• Population size is affected by abiotic factors (density-independent) and biotic factors (density-dependent: competition, predation, disease).
• Succession is the progressive change in community composition: pioneer species → seral stages → climax community.
• Primary succession starts on bare surfaces; secondary succession starts where soil exists.
• Simpson's Index ($D = 1 - \sum(n/N)^2$) quantifies biodiversity; higher $D$ = greater diversity.